The administration of testosterone (T) can reversibly suppress spermatogenesis in men. However, although azoospermia is achieved in a majority of men, significant numbers typically maintain high enough sperm production to pose a risk of initiating a pregnancy. A significantly higher prevalence of azoospermia has been noted in Asians than in Caucasians. The failure of individuals or groups of men to become azoospermic diminishes the promise of hormone-based contraception. The long-term goal of the proposed research is to determine the biological mechanisms that explain why some men become azoospermic while others do not. We will take advantage of the naturally occurring ethnic differences, namely the response of Asians and Caucasians, to T-based contraception to address this biological issue. Three specific aims are proposed. In the first, we hypothesjze that differences in T metabolites (e.g. lower DHT levels) in Asian than in Caucasian men might help explain the increased responsiveness of Asians to T-based contraception. To address this hypothesis, we first will quantify T and DHT present within the testes of untreated fertile Asian and Caucasian men, using a highly sensitive liquid chromatography tandem mass spectroscopy method (LC/MS/MS) to do so. A second objective will be to compare the total intratesticular bioactive androgen concentrations in Asian and Caucasian men, using a highly sensitive assay of androgen bioactivity based upon stimulation of androgen receptor transcriptional activity in mammalian cells. The central hypothesis to be tested is that a greater percentage of androgen is bound and therefore not bioavailable in Asian than in Caucasian men. If true, this would suggest that lesser reduction of T and DHT would be required to achieve azoospermia in Asian than in Caucasian men. In the second aim, we will determine whether there are differences in the bioactive, bioavailable pool of androgens within the testes'of Asians and Caucasians following the men receiving a contraceptive dose of T. The central hypothesis that we will test is that in response to a contraceptive T dose, the Caucasian men;will have greater amounts of intratesticular bioactive androgens than Asian men. In the third aim, we will test the hypothesis that at specific total intratesticular T concentrations, there will be less bioavailable, bioactive androgen in the testes of Asian than Caucasian men. The results of these studies should be invaluable to the development of a safe and effect hormonebased male contraceptive. Moreover, comparing two groups with different sensitivities to a contraceptive dose of T should provide significant insight into the physiological mechanisms in men through which T regulates spermatogenesis.